1. Field of the Invention
The invention relates to a video signal processing apparatus having a function for multiplexing a data signal to a video signal and transmitting the resultant signal.
2. Related Background Art
In recent years, a television conference system is shifting from a large scale system to a small scale and personal system such as a desk-top type system or the like. A video camera which is used as input means of an image in the above television conference system, a monitoring camera, or the like is also shifting to a camera of a small size and a low price.
In the field of the small video camera, therefore, a video camera of a camera head separating type is a main stream. Namely, it has a form such that an image pickup unit and a signal processing unit are separated through a cable. A camera head serving as an image pickup unit is constructed by an image pickup device such as a CCD (Charge Coupled Device) or the like and simple processing circuits such as sample and hold circuit, gain control circuit, and the like, thereby realizing a miniaturization. A signal processing board serving as a signal processing unit is constructed by a video signal processing circuit, a white balance control circuit, an image data compressing circuit, an interface circuit with a personal computer (hereinafter, simply abbreviated to a "PC"), and the like, thereby realizing a miniaturization.
FIG. 1 is comprised of FIGS. 1A and 1B showing constructional block diagrams of a desk-top type television conference system using such a video camera of the camera head separating type. As shown in FIGS. 1A and 1B, the desk-top type television conference system is constructed by a camera head 100, a signal processing board 200, an electric tripod 300, and cables 150 and 160 for connecting them.
In the desk-top type television conference system shown in FIGS. 1A and 1B, first, processes in case of transmitting video and audio signals to a communication line will now be described.
In the camera head 100, an image is formed on an image pickup surface of an image pickup device (CCD) 103 through an image pickup optical system constructed by a pickup lens 101, an iris mechanism 102, and the like. A video signal corresponding to a video image formed on the image pickup surface is generated from the CCD 103. In a preprocessing circuit 104, preprocesses such as CDS (Correlation Double Sampling) process, AGC (Auto Gain Control) process, and the like are executed to the video signal. After that, the processed signal is transmitted to the signal processing board 200 through a multiplexing circuit 105 and the cable 150. In the signal processing board 200, the video signal that is supplied from the camera head 100 is converted into a digital signal by an analog/digital (A/D) converter 201. A color process and the like are executed in a state of the digital signal by a video signal processing circuit 202. After that, the signal is compressed in an encoder and decoder 203 together with audio data which is inputted from an outside The compressed signal is transmitted to a digital communication line such as an ISDN or the like through a communication interface 204.
On the other hand, processes in case of receiving data regarding video and audio signals transmitted through the communication line will now be described.
Data that is sent from the digital communication line such as an ISDN or the like is inputted to the signal processing board 200 through the communication interface 204 and is contrarily decompressed by the encoder and decoder 203. The decompressed video data and audio data are transferred to the PC via a bus interface 205 and are displayed and reproduced by a display apparatus and a speaker apparatus of the PC.
On the other hand, in order to move a photographing position of the camera head 100, the camera head 100 is attached onto the electric tripod 300. The electric tripod 300 is constructed in a manner such that the photographing position of the camera head 100 can be moved by making a pan motor 302 and a tilt motor 303 operative by an MPU (Micro Processor Unit) 301 in accordance with instructions of a pan operation and a tilt operation by control data that is supplied from the PC through the signal processing board 200, cable 150, camera head 100, and cable 160.
In the desk-top type television conference system using the video camera of the camera head separating type as mentioned above, in case of executing the camera control (namely, a focusing control, a zooming, an iris control, an electronic shutter control, a gain control, and the like) in the camera head 100 and the photographing position control (namely, pan operation, tilt operation) by the electric tripod, since the camera head 100 and signal processing board 200 or the electric tripod 300 and the PC are separated, various control data has to be communicated between an MPU 106 of the camera head 100 and an MPU 206 of the signal processing board 200.
As a cable which is provided between the camera head 100 and signal processing board 200 and is used to connect them, it is desirable that the number of wirings is as small as possible from a viewpoint of operability. Therefore, a method of reducing the number of wirings of the cable by multiplexing control data which is communicated between the camera head 100 and signal processing board 200 to the video signal and transmitting the multiplexed signal is used.
Namely, in the video camera of the camera head separating type, when the control data is transmitted from the signal processing board 200 to the camera head 100, the control data is transmitted via an exclusive-use cable. When the control data is transmitted from the camera head 100 to the signal processing board 200, on the other hand, by using a method called a VIDS (Vertical Interval Data Signal), the control data is multiplexed to the video signal and the multiplexed signal is transmitted through a cable for transmitting the video signal.
FIG. 2 shows a multiplexing period of time of the control data in case of using the above method. As shown in FIG. 2, the control data is added with an error check code by a binarization signal form of an NRZ and in this state, the control data is multiplexed to, for example, each horizontal scanning period of the video signal of a vertical blanking period (V blanking period).
Hitherto, in case of multiplexing the VIDS to the video signal as mentioned above, the multiplexing circuit 105 of the camera head 100 is constructed by using an operational amplifier. The multiplexing operation in the multiplexing circuit 105 using the operational amplifier will now be described with reference to time charts of FIGS. 3A, 3B, 3C and 3D.
In FIGS. 1A and 1B, the control data which is generated from the MPU 106 is converted into a binary signal (refer to FIG. 3A) in an encoding and decoding unit 107 and is, further, adjusted to a binary signal (VIDS) (refer to FIG. 3C) of a level that is actually multiplexed and is outputted to the multiplexing circuit 105 for, for example, the V blanking period of the video signal in accordance with a timing signal which is generated from a timing signal generator 108. The video signal (refer to FIG. 3A) which is supplied from the preprocessing circuit 104 and the level converted binary signal (refer to FIG. 3C) which is outputted from the encoding and decoding unit 107 are added by the multiplexing circuit 105 comprising a wide band operational amplifier. A resultant addition signal is outputted as a video and control data multiplexed signal (refer to FIG. 3D) to the signal processing board 200 through the cable 150.
The video and control data multiplexed signal which is supplied from the camera board 100 is converted into the digital signal by the A/D converter 201 of the signal processing board 200. The digital signal is supplied to both of the video signal processing circuit 202 and an encoding and decoding unit 207.
The encoding and decoding unit 207 separates and decodes the control data from the digital video and control data multiplexed signal which is supplied from the A/D converter 201. After that, the encoding and decoding unit 207 supplies the control data to the MPU 206 or encodes the control data which is outputted from the MPU 206 and supplies to the encoding and decoding unit 107 of the camera head 100 through a control data exclusive-use line of the cable 150.
In the above conventional system, however, since the control data is multiplexed by constructing as mentioned above, a number of parts such as operational amplifier, level control circuit, peripheral circuits of those circuits, and the like are necessary to construct the encoding and decoding unit 107 and multiplexing circuit 105, so that the circuit is complicated and the scale is large. Since the multiplexing circuit for multiplexing the control data needs to use the wide band operational amplifier (in case of the video signal, a band of about 100 MHz is necessary), there is a problem such that the costs of the system are high.